1. Field of the Invention
The present invention relates to a power steering system for a motor vehicle, and more particularly, the present invention relates to a power steering system for a motor vehicle, which increases and decreases, upon being actuated, steering force by using electromagnetic force in a manner such that driver steering effort varies depending upon a speed of the motor vehicle.
2. Description of the Related Art
Generally, a conventional hydraulic power steering system for a motor vehicle, which utilizes a hydraulic actuator as a steering assistant force source, includes an output shaft which is connected to steerable wheels of the vehicle and a torsion bar for an input shaft which is connected to a steering wheel. The output shaft and the torsion bar are coaxially connected with each other. In the case that steering torque is exerted to the steering wheel by a driver, relative angular displacement is resulted in between the input shaft and the output shaft, in relation to torsion of the torsion bar, which is caused by the exertion of the steering torque. At this time, depending upon the relative angular displacement, by the rotation of a rotary valve which is arranged between the input shaft and a valve body, fluid pressure is supplied to a working chamber which corresponds to a steering direction, thereby generating steering assistant force.
Steering assistant force which is generated in this way, is related with steering torque and torsional displacement of the torsion bar. FIG. 1 is a graph illustrating a relationship between steering torque and torsional displacement of a torsion bar in a power steering system for a motor vehicle. From FIG. 1, it can be readily seen that, as torsional displacement of a torsion bar is increased, steering torque is also increased in proportion to the torsional displacement of the torsion bar. Therefore, because control characteristics are determined only by the torsional displacement of the torsion bar, a proportional relationship as shown in FIG. 2 is established between steering torque which is exerted to a steering wheel and steering assistant force generated by fluid pressure which is supplied to a hydraulic cylinder.
Referring to FIG. 2, there is shown a graph illustrating a relationship between steering torque and fluid pressure in a power steering system for a motor vehicle. When observing a change in fluid pressure in relation to steering torque, it is to be readily understood that, in a lowered steering torque range `b`, fluid pressure is low and is gently increased, and, in an elevated steering torque range `a`, fluid pressure is abruptly increased. Hence, when parking a vehicle or running at a low speed, because large steering assistant force is required, control characteristics of the elevated steering torque range `a` in which fluid pressure is abruptly increased, are adopted. Also, when running at a medium speed or at a high speed, control characteristics of the lowered steering torque range `b` in which fluid pressure is low, are adopted so that running stability is secured and the vehicle is steered with small steering assistant force.
In the meanwhile, in the case that torque value of the steering wheel is lightly established on the basis of the parking situation or the low speed running situation, because the vehicle is steered even with very small torque in the high speed running situation, a problem is caused in that straight-ahead running stability of the vehicle in the high speed running situation is deteriorated. On the contrary, in the case that torque value of the steering wheel is heavily established on the basis of the high speed running situation, a problem is caused in that it is impossible to obtain sufficient steering assistant force in the parking situation or the low speed running situation.
To cope with these problems, a vehicle speed-responsive, resistant force-generating apparatus has been developed. In the vehicle speed-responsive, resistant force-generating apparatus, depending upon a speed of a vehicle, steering torque is altered in relation to torsional displacement, and fluid pressure is altered in relation to steering torque, whereby, in a low speed running situation, large steering assistant force is achieved with small steering torque and, in a high speed running situation, steering resistant force is increased to improve running stability.
As the vehicle speed-responsive, resistant force-generating apparatus, a hydraulic resistant force-generating apparatus and an electromagnetic resistant force-generating apparatus are disclosed in the art. The hydraulic, resistant force-generating apparatus uses reaction force due to fluid pressure of a fluid flow controlling apparatus which increases a fluid supply amount when a vehicle is running at a low speed and decreases a fluid supply amount when the vehicle is running at a high speed. The hydraulic, resistant force-generating apparatus also uses mechanical reaction force of a valve. The electromagnetic, resistant force-generating apparatus alters resistant force by using electromagnetic force.
FIG. 3 is a partially broken-away perspective view illustrating an electromagnetic mechanism of an electromagnetic, resistant force-generating apparatus of a conventional power steering system for a motor vehicle.
The electromagnetic, resistant force-generating apparatus 92 as shown in FIG. 3, which is disclosed in U.S. Pat. No. 5,119,898, includes an encapsulated annular exciting coil 130, a cylindrical permanent magnet ring element 104, and a pair of rotary ferromagnetic pole pieces 106 and 108. The pair of rotary ferromagnetic pole pieces 106 and 108 are formed with a plurality of outwardly facing teeth 120 and a plurality of inwardly facing teeth 122, respectively. The cylindrical permanent magnet ring element 104 is secured to a rotor hub 110 which supports a valve body through a sleeve bearing in a manner such that the valve body can be rotated. The rotary ferromagnetic pole piece 106 which faces the cylindrical permanent magnet ring element 104, is disposed in a manner such that it can be rotated along with an elongated cylindrical spool shaft.
At this time, the cylindrical permanent magnet ring element 104 is formed in a manner such that radially magnetized polarities thereof are repeatedly alternated along a circumferential direction.
FIG. 4 is a schematic view illustrating magnetic flux of a rotary magnetic circuit of the electromagnetic mechanism of FIG. 3.
Here, FIG. 4A represents a state wherein centered alignment of magnetic paths is effected, and FIG. 4B represents a state wherein centered alignment of magnetic paths is distorted due to the movement of the cylindrical permanent magnet ring element 104.
In the case that steering torque is not exerted to a steering wheel, magnetic paths which are induced by the encapsulated annular exciting coil 130 are, as shown in FIG. 4A, maintained in a state wherein they are centered between the pair of rotary ferromagnetic pole pieces 106 and 108. Then, if steering torque is exerted to the steering wheel, the cylindrical permanent magnet ring element 104 is rotated relying upon a rotating condition of the valve body, and thereby, as shown in FIG. 4B, centered alignment of magnetic paths is distorted. Due to the magnetic paths which are distorted in this course, as a centered alignment restoring phenomenon occurs, restoring force for returning the cylindrical permanent magnet ring element 104 to its original position is generated, and this restoring force acts as resistant force to steering assistant force.
In this way, by modifying a magnitude of the centered alignment restoring force of the magnetic paths which are induced by the encapsulated annular exciting coil 130, it is possible to regulate steering assistant force depending upon a speed of a vehicle.
However, the electromagnetic, resistant force-generating apparatus 92 according to the conventional art, constructed as mentioned above, suffers from defects in that, since the cylindrical permanent magnet ring element 104 which is formed in a manner such that radially magnetized polarities thereof are repeatedly alternated along the circumferential direction, is intervened between the pair of rotary ferromagnetic pole pieces 106 and 108 for rendering hydraulic actuation of the power steering system and the centered alignment restoring force of the magnetic paths which are induced by the encapsulated annular exciting coil 130, and acts to return the cylindrical permanent magnet ring element 104 to its original position, it is difficult to magnetize the cylindrical permanent magnet ring element 104 in such a manner that polarities thereof are repeatedly alternated along the circumferential direction as described above, whereby productivity is deteriorated.
Further, a power steering apparatus for a motor vehicle with a two piece inner pole member and a method for making the same are disclosed in U.S. Pat. No. 5,931,195 which is issued to Joel Edward Birsching et al. on Aug. 3, 1999 and assigned to General Motors Corporation.
The electromagnetic apparatus disclosed in the '195 patent is provided to vary a magnitude of effective restoring torque of a torsion bar. The electromagnetic apparatus comprises a stationary exciting coil, an inner pole member possessing an extension of a cylindrical valve member having a plurality of outwardly facing teeth, and an outer pole member fixed on a spool shaft for rotation therewith and having an equal plurality of opposing inwardly facing teeth. Here, the inner pole member comprises a hub member formed in an annular cylindrical configuration extending through the coil and made of a first magnetic material having a high magnetization for a given coil current for maximum flux through the coil, and further comprises a tooth member possessing the plurality of outwardly facing teeth and formed of a second magnetic material having a smaller magnetization than the first magnetic material for the given coil current.
However, the electromagnetic apparatus disclosed in the '195 patent, constructed as mentioned above, still encounters a problem in that, since the inner pole member is formed by two magnetic materials having different magnetizations, resistant force is not generated, but only attractive force is generated, and steering force is regulated using this attractive force. Therefore, a regulating range of a default value is reduced to a half when compared to a structure using a permanent magnet. As a result, it is only possible to control steering force in a manner such that steering force is enlarged, that is, the steering wheel is rotated with heavy effort, and it is impossible to control steering force in a manner such that steering force is diminished, that is, the steering wheel is rotated with light effort, whereby satisfiable steering assistant force cannot be accomplished.